1. Field of the Invention
The present invention is related to an online diagnostic valve monitoring system and, more particularly, to a system for monitoring valves in a complex piping network such as that found in an electrical power generating station.
2. Description of the Related Art
Recent studies funded by the Electrical Power Research Institute (EPRI) and the Nuclear Regulatory Commission (NRC) have shown that both nuclear and fossil fuel electrical generating plants are subjected to a significant number of shutdowns or outages due to valve failures, despite the fact that 30% to 50% of annual plant maintenance is devoted to valves. Other facilities such as chemical processing plants, pulp and paper plants, off shore platforms, orbiting satellite power systems and almost any nuclear powered device, including nuclear submarines, have complex piping systems which can be disrupted by valve failures. The complexity of such systems make the likelihood of valve failure during a relatively short period of time virtually inevitable.
For example, a typical electrical power plant may have 5,000 to 10,000 valves. Of these, approximately 5% to 10% are critical to maintaining production of electricity at the plant. Despite maintenance efforts, it is common for forced outages to occur every 3 to 4 months due to valve malfunctions. Furthermore, when a large valve fails, the outage may last for over three weeks.
There have been numerous efforts to avoid this situation. In order to minimize the likelihood of causing an outage, valve manufacturers commonly give extremely conservative estimates of valve life and service intervals. Electrical power companies have responded by scheduling valve maintenance or replacement which is performed regardless of whether there is any problem with the valve. These two factors combine to result in excessively high maintenance costs. Typically, 30% of annual plant maintenance is devoted to valves and at some plants as much as 50% of all maintenance is valve-related.
Despite these efforts, outages caused by valves continue to occur. A major southeast utility is recognized as having one of the best maintained nuclear power plants in the country. Yet in the early 80's, this plant experienced two weeks of forced outages per year due to valve-related causes.
There have been attempts to reduce the high cost of unnecessary maintenance while reducing forced outages caused by valve malfunctions. One attempt has been to schedule maintenance based on the statistical history of valve failures for particular valves and particular locations in the piping system. Other efforts have been directed to testing valves to determine when maintenance is required. Several different systems which attempt to meet this need are described in the November 1985 Interim Report EPRI NP-4254 for Project 2233-2.
One technique summarized in this EPRI report is a system which measures motor current of a motor operated valve (MOV), the tripping of control switches and movement of the "spring pack" which indirectly measures valve stem thrust. The conventional equipment used to make these measurements is capable of measuring one valve at a time per measuring device and requires that the equipment be connected only temporarily. The signal traces which result from the sensors are recorded and analyzed by a human expert to determine whether the valve is adjusted and operating properly. As a result, when the valve is determined to be in need of maintenance, a decision must be made whether to extend the outage for the immediate repair of the valve or whether to wait until the next outage (planned or unplanned) occurs. This system is described in more detail in publication 84-NE-16 of the American Society of Mechanical Engineers (ASME) and in U.S. Pat. No. 4,542,649, issued Sept. 24,1985 to Charbonneau et al. This system is claimed to be capable of detecting mechanical wear in the valve or motor operator, improperly adjusted control switches, degradation of the motor's electrical system and improper packing or lubrication of the valve.
While the diagnostics provided by this system are useful, there are significant drawbacks. The indirect sensing of stem strain using the spring pack of a motor operated valve provides less information than direct sensing and introduces the likelihood of error. With 250 to 1,000 critical valves in a typical power plant, the time required to set up and test each of the critical valves during an outage is prohibitive. Either a large number of test stations and manpower must be used or the outage must last for a long period of time; otherwise, only a portion of the critical valves are tested at each outage. In addition, the time required for analysis by a human expert inevitably results in further delays in ending the outage or in repairing a malfunctioning valve.
As a result of these drawbacks, EPRI Project 2233-2 involved the use of a testable microprocessor control system in motor operated valves. Such a system provides an increased amount of information about the operation of the motor, but little information is provided about the mechanical operation of the valve. Therefore, an ultrasonic sensor was used in the EPRI project to sense stem elongation. The use of such sensors, like the first system described above, is unable to provide all of the information which would be useful. In addition, the EPRI project did not address the problems of data collection and data analysis for a large number of valves.
The long-felt need for an improved method of valve diagnosis for complex piping systems has prompted the development of other techniques. For example, analysis of motor current signals has been undertaken by Haynes and Eissenberg at Oak Ridge National Laboratory (ORNL). According to the U.S. Department of Energy, this work has resulted in U.S. patent application Ser. No. 06/913,193, incorporated herein by reference. Copies of the '193 application are available from DOE and thus, these techniques are known as of the filing of the subject application. As a result, the contents of the '193 application can be considered as describing the knowledge of one of ordinary skill in the art of analyzing motor current traces.
The large number of valves which are critical to a power plant's operation and the large number of valves which require maintenance make the prior art "one at a time" systems extremely inefficient. Substantial amounts of time and money are required to perform the tests on each valve and then the data must be analyzed by a human expert one valve at a time. These problems are not unique to power plants, but can be found in almost any complex piping system having large number of valves including petrochemical plants, pulp and paper plants and virtually all applications of nuclear power. When techniques are used that require test equipment to be brought to the site of the valve near a nuclear reactor, the maintenance personnel may also be exposed to unnecessary radiation.